This invention relates to the field of medical diagnosis, and more specifically, to a method and apparatus for noninvasive measurement of arterial compliance.
High blood pressure and hardening of the arteries can lead to coronary problems. Obtaining various measurements of the vascular system, including compliance of large and small vessels, and systemic resistance, provides physicians with information useful in diagnosing and treating early stages of coronary disease.
U.S. Pat. No. 6,017,313 (xe2x80x9cthe ""313 patentxe2x80x9d) to Christopher W. Bratteli et al. (incorporated herein by reference) discloses an apparatus and method for blood pressure pulse waveform contour analysis. Methods of processing an arterial blood pressure waveform to extract clinically useful information on the state of the cardiovascular system are disclosed. In order to obtain the parameters of the modified Windkessel model, the diastolic portion of a subject""s blood pressure waveform is scanned over a plurality of ranges and the range that produces the best fit of data and lowest error estimates are selected.
U.S. Pat. No. 5,211,177 (xe2x80x9cthe ""177 patentxe2x80x9d) to Charles F. Chesney et al. (incorporated herein by reference) discloses method and apparatus for measuring properties of the human vasculature using an electrical analog model of vascular impedance. These properties include the compliance of large and small vessels, and systemic resistance. These measurements and others obtained from the model can, in turn, be used to diagnose states of health or disease, and to assess the effectiveness of treatment regimes. For example, see Finkelstein, S. M., Collins, V. R., Cohn, J. N., Arterial vascular compliance response to vasodilators by Fourier and pulse contour analysis, Hypertension 1988:12:380-387, the entire disclosure of which is incorporated herein by reference.
U.S. Pat. No. 6,159,166 (xe2x80x9cthe ""166 patentxe2x80x9d) to Charles F. Chesney et al., and entitled SENSOR AND METHOD FOR SENSING ARTERIAL PULSE PRESSURE (incorporated herein by reference) discloses a method and apparatus useful for tonometric measuring of an arterial pulse pressure waveform. U.S. Pat. No. 6,331,161 to Charles F. Chesney et al., and entitled METHOD AND APPARATUS FOR FABRICATING A PRESSURE-WAVE SENSOR WITH A LEVELING SUPPORT ELEMENT (incorporated herein by reference) discloses a method and apparatus of a sensor useful for tonometric measuring of an arterial pulse pressure waveform. U.S. Pat. No. 6,132,383 to Charles F. Chesney et al., and entitled APPARATUS AND METHOD FOR HOLDING AND POSITIONING AN ARTERIAL PULSE PRESSURE SENSOR (incorporated herein by reference) discloses a method and a wrist brace and sensor holder useful with any of the above sensors for tonometric measuring of an arterial pulse pressure waveform.
U.S. Pat. No. 5,241,966 (xe2x80x9cthe ""966 patentxe2x80x9d) to Stanley M. Finkelstein; et al. issued Sep. 7, 1993 (incorporated herein by reference) discloses an apparatus for measuring stroke volume/cardiac output that includes a transducer for measuring arterial blood pressure waveform, a digitizer for digitizing the analog signal generated by the transducer and a digital signal processor for determining ejection time and heart rate. Processor circuitry determines cardiac output using the ejection time, heart rate, the body surface area and age of the patient.
Compliance is a fundamental property of any pressure-volume system such as the arteries. In the arteries, reduced compliance has been offered as a mechanism whereby the work load on the heart is increased and myocardial perfusion is decreased, thus leading to cardiac disease. It is also thought that disease of the arteries themselves may be evaluated by measurement of arterial compliance. Therefore, arterial compliance is an important cardiovascular parameter.
What is needed is improved noninvasive measurement of arterial compliance.
The present invention provides improved noninvasive measurement of arterial compliance using a combination of noninvasive arterial tonometry and noninvasive cuff oscillometry. Described are a number of improved approaches to estimating systemic vascular resistance and/or compliance. In one embodiment, a computer-controlled pneumatic cuff having a sensor coupled to sense pressure and pressure variations in the cuff is used to obtain oscillometric signals (e.g., from an upper arm of a patient), and simultaneously a contact-pressure sensor (e.g., some type of microphone) is used to obtain tonometric signals (e.g., from the contralateral wrist of the patient and, e.g., a transducer placed on the radial artery), and the oscillometric signals and tonometric signals, which are correlated to one another in time, are combined to calibrate one another and/or obtain information about the patient not available conventionally. In some embodiments, the oscillometric measurement is used to calibrate the tonometric measurement.
One aspect of the present invention provides a method to compute arterial compliance from oscillometric data (2nd embodiment listed below). This provides a compliance measurement that is distinct from the compliance measurement described in the ""177 patent.
Another aspect of the present invention provides a method for improving pressure calibration from oscillometric data (1st and 3rd embodiments listed below). This can be used to improve the above measurement, to improve the ""177 patent""s described compliance measurement, or to improve almost any measurement that uses arterial pressures.
Yet another aspect of the present invention provides a set of methods for building models that compute flow from pressure data (with or without oscillometric data). This can be used in the same way as the cardiac output model described in the ""313 patent (i.e., it can be used to improve the compliance estimate).
The present invention provides a number of improvements or refinements to the approaches to waveform analysis set forth in the ""177 and the ""313 patents. These improvements include an accurate calibration method and apparatus for using an oscillometric signal to calibrate the pressures of tonometric signals in the contralateral arterial site. These improvements also include a simple uncorrected volume arterial compliance curve that is obtained by plotting relative arterial volume under the cuff against brachial arterial transmural pressure. Further improvements account for a bias (i.e., an overestimation of the in vivo compliance curve for the arterial segment due to shear stresses at the ends of the cuff) by providing a correction for the transmural pressure that is based on stress-strain properties of the upper arm.
In some embodiments, the method of the ""313 patent is improved with use of the SVR model of the present invention.